1. Field of the Invention
The present invention relates to a color processing method and a color processing apparatus for transforming a color signal in a target color space such as L*a*b* or RGB into a four-color signal including black, a storage medium for storing a program for executing the color processing method, and furthermore, a color transforming apparatus and an image forming apparatus utilizing a result of a transformation carried out by the color processing method and the color processing apparatus.
2. Description of the Related Art
When a color image is color-printed by an electrophotographic method, four-color printing using yellow (Y), magenta (M), cyan (C) and black (K) is usually utilized very often. On the other hand, a color signal on a three-dimensional color space such as L*a*b* and L*u*v* color spaces, which do not depend on a device or an RGB color space used in a monitor signal, is often used for a general color signal. When a color image is color printed, accordingly, it is necessary to transform the color signal on the three-dimensional color space into a four-dimensional color space. However, since this transformation is carried out between different dimensions, it does not correspond one-on-one. There are a plurality of combinations of the color signal on the three-dimensional color space and a four-color signal reproducing the color signal.
As a method for determining the combination of the color signal on the three-dimensional color space and the four-color signal on the four-dimensional color space, the component of black is calculated from Y, M and C and black (K) is then added by foundation color removal. As another method, black (K) is first determined by some method and amounts of Y, M and C corresponding to the amount of black (K) are determined. Recently, the latter method has been a mainstream in respect of a color reproducibility and various methods have been tried.
For example, JP-A-5-292306 has disclosed a method in which preset weighting is first carried out over the K amount (achromatic black colorant amount) of the four-color signal reproducing a target color signal under the conditions of Y=0%, M=0% or C=0%, thereby determining a new K amount. Then, the amounts of Y, M and C reproducing a target color signal are determined according to the K amount. Consequently, a color reproduction can be realized with high precision, and at the same time, the amount of black can be controlled in response to the purpose.
In a method disclosed in JP-A-6-242523, moreover, a K amount (maximum black colorant amount) of a four-color signal reproducing a target color signal is first calculated under conditions of Y=0%, M=0%, C=0% or K=100%. Moreover, a K amount (minimum black colorant amount) of a four-color signal reproducing a target color signal is calculated under conditions of Y=100%, M=100%, C=100% or K=0%. By using the maximum and minimum black colorant amounts thus calculated, a new K amount is determined by a preset parameter therebetween and the new amounts of Y, M and C reproducing the target color signal are determined according to the K amount. Consequently, it is possible to use, at a maximum, a color gamut, which can be reproduced by four colors including black.
A condition of a coverage limitation is imposed to a general output device. The coverage limitation implies that an upper limit is set to the total amount of a recording material such as a toner or an ink, which is used for reproducing a color signal. Mainly, the coverage limitation is used for reducing a deterioration in a reproducing performance caused by excessively utilizing the recording material such as a toner or an ink or in the bulge of a print surface, or for protecting the output device.
In all methods according to the related art, however, the coverage limitation is not taken into consideration. For this reason, in some cases in which the amount of black is controlled by the preset parameter, the reproduction cannot be carried out irrespective of a reproducible color gamut. More specifically, the total amounts of the Y, M, C and K thus calculated depart from the scope of the coverage limitation. As a result, a color gamut compression is caused so that color reproducing precision is deteriorated.
In order to solve such a problem, in Japanese Patent Application No. 2001-34867, for example, an optimum black colorant amount is calculated in a color gamut, which can be reproduced by three colors, and the outermost contour surface of a four-color gamut satisfying the coverage limitation is searched from the contour of the color gamut, which can be reproduced by three colors, and K is determined by using the black colorant amount on the outermost contour surface and the optimum black colorant amount calculated previously. By using the calculated K, thus, it is possible to obtain a combination of Y, M, C and K to satisfy the coverage limitation.
In general, the design of the optimum black colorant amount is usually set to be decreased with an increase in a color saturation component of a target color signal. The reason is that the color becomes dull by a mixture of the black component. By decreasing the black colorant amount with an increase in the color saturation, a natural color reproduction can be carried out. In the black colorant amount obtained by the method disclosed in the Japanese Patent Application No. 2001-34867, however, the black colorant amount in the three-color gamut is regulated as described above and the black colorant amount in the outermost contour of the color gamut satisfying the coverage limitation is the maximum black colorant amount. In an achromatic color, therefore, it is possible to sufficiently use up the color gamut to satisfy the coverage limitation by the Y, M, C and K thus calculated, and the black colorant amount is to be increased with an increase in the color saturation component. As a result, a color signal to be reproduced becomes unnatural in some cases.